The present invention relates to a casing for a frame-supported pellicle which is a device used for the protection of a photomask or reticle during a photolithographic patterning procedure in the manufacture of various kinds of fine electronic devices such as ICs, LSIs, liquid-crystal display panels and the like by preventing falling and deposition of dust particles thereon.
As is well known, most of the manufacturing processes of fine electronic devices such as ICs, LSIs, liquid-crystal display panels and the like involve a photolithographic patterning work in which a photoresist layer formed on the surface of a substrate such as a semiconductor silicon wafer is patternwise exposed to ultraviolet light through a transparency called a photomask bearing a pattern to be reproduced in the photoresist layer. In view of the extreme fineness of the pattern to be reproduced, it is very important that the photomask is absolutely free from dust particles deposited thereon because deposition of any smallest number of dust particles on the photomask may cause serious degradation in the quality of pattern reproduction due to scattering of light by the dust particles.
Because an absolutely dust-free condition can never be accomplished even in a clean room of the highest class, it is a usual procedure that the photomask is protected against falling dust particles by mounting thereon a frame-supported pellicle which is an integral device consisting of a rigid frame of aluminum and the like, referred to as a pellicle frame hereinafter, and a highly transparent thin film of a plastic resin, referred to as a pellicle membrane hereinafter, spread over and adhesively bonded to one end surface of the pellicle frame in a slack-free fashion. Since the pellicle membrane is above the photomask by the height of the pellicle frame when the frame-supported pellicle is mounted on a photomask, dust particles deposited on the pellicle membrane have no particular adverse influences on the quality of pattern reproduction by the ultraviolet light focused on the photomask. Needless to say, nevertheless, it is very important that a frame-supported pellicle is kept as dust-free as possible insofar as it is used for the dustproof protection. Therefore, it is usual that a frame-supported pellicle is encased in an air-tight casing during storage and transportation in order to prevent deposition of dust particles thereon.
Among the various types of casings proposed heretofore and under practical use, FIGS. 1 and 2 illustrate a typical casing by a plan view and by a vertical cross sectional view, respectively, consisting of a body of casing 10 and a covering 20 encasing a frame-supported rectangular pellicle consisting of a pellicle frame 31 and a pellicle membrane 30 spread over and adhesively bonded to the upper end surface of the pellicle frame 31 in a slack-free fashion. The body of the casing 10 is provided along the periphery with an upwardly protruded rib 11 for the purpose of engagement with the covering 20 and the center area 12 of the body 10 is raised in the form of a rectangular stage to be wide enough for mounting the pellicle frame 31 forming a recess 12a on the lower surface of the body 10. Surrounding the area 12 for mounting of the pellicle frame 31, a plural number of pin protrusions 13 are integrally formed to provide a means for engagement of the pellicle frame 31. Though not essential, a plural number of supporting legs 19 are formed on the lower surface of the casing body 10.
On the other hand, the covering 20 has the periphery upwardly bent to form a U-shaped turning 21 which comes into elastic engagement with the rib 11 around the casing body 10 when the covering 20 is mounted on the casing body 10. Further, stepwise zone 22 surrounding the center area of the covering 20 serves as a presser for the frame-supported pellicle mounted on the mounting stage 12 surrounded by the pin protrusions 13 by coming into contact with the upper end of the pellicle frame 31.
While the above mentioned casing body 10 and the covering 20 of the casing for a frame-supported pellicle are made usually from a plastic resin, it is important that the surface of the casing body and covering per se is also antistatic in order to prevent dust particle deposition thereon due to electrostatic attraction because dust particles deposited thereon may eventually migrate on to the frame-supported pellicle encased in the casing during transportation to cause troubles. As is well known, the surface of a plastic resin-made body can be rendered antistatic either by compounding the plastic resin with an antistatic agent such as surface active agents or by coating the surface with a coating composition containing an antistatic agent. These prior art methods are not always quite satisfactory because the antistatic nature of the surface can be readily lost when the body is rinsed with water or cleaned by using a cloth. When the casing is shaped from a plastic resin composition containing a surface active agent, moreover, it is sometimes the case in the lapse of time that the surface active agent contained in the plastic resin composition bleeds on the surface of the casing to cause contamination of the frame-supported pellicle encased in the casing. As a means to ensure semi-permanency of the antistatic performance in a plastic resin composition without the above mentioned problems, proposals have been made for a method in which the plastic resin is compounded with a particulate electroconductive material such as carbon black. While effective to obtain surface conductivity of a plastic resin body, however, this method has a contrary effect in respect of dust particle occurrence because dust particles are eventually produced from the carbon black incorporated into the plastic resin in such a large amount as to ensure electroconductivity of the body shaped from the resin composition.